Devices and methods for detecting the position of a sheet or of any printing support on a printing plane in a silk-screen printing apparatus have been known for some time.
In particular, European patent application No. 0126723 describes an optical device for detecting the position of the registration marks of a sheet or another printing support which comprises two TV cameras, each supported at the end of extendable elements, such as pneumatic pistons, which move them from a region outside the printing plane above the latter and then to the printing support. By means of the two TV cameras, the position of the registration marks, provided on the printing support, is detected, and according to their displacement relatively to the exact position which they should assume if the printing support were positioned correctly on the printing plane, a printing frame is orientated so as to eliminate any error of position of the printing support on the printing plane.
The device of this European application significantly improves the precision with which the offset between the real position and the theoretical position of the printing support on the printing plane is eliminated, is characterized by the fact that the weight of the TV cameras causes a rapid wear of the extendable elements so that they increasingly tend to no longer move the respective TV cameras to the same reading point, especially after a number of operating cycles.
This imprecision in the positioning of the TV camera is also due to the inertia generated by the mass of the TV camera during its motion.
Moreover, in order to contain maximally the disadvantages described above, the supporting elements of the TV cameras must be provided with high precision mechanics, which entails an increase in the costs of manufacture of the optical reading device, and the TV cameras are provided with sophisticated electronics and are produced specifically for this application.
Thus, the detection error made by the optical reading device of the European patent application is exactly equal to a displacement defined by the distance between the theoretical point at which the TV camera should stop to detect the registration marks on the printing support and the real point to which the TV camera, for the reasons described above, is actually moved by the extendable elements.
It is also known that the materials to be printed by silk-screen printing apparata are often unstable.
For these reasons, the dimensions of a product often vary after printing, aging, drying, washing, etching and stacking.
It is therefore very important to be able to perform a comparison of the work cycles subsequent to the first one, therewith or with an original model considered as a sample.
Hence it is necessary to adjust the silk-screen printing apparatus many times during the various work cycles, as well as to prepare and adjust the same at the beginning of the first work cycle, with a significant waste of time with disadvantageous management cost and idle times.
By means of the prior-art optical reading devices for detecting registration marks, only the registration marks of the printing support on the printing plane are detected, so that a first adjustment of the printing frame, according to the displacement of its registration mark with respect to the registration marks of the printing support, is performed manually according to the experience of the operator which detects said displacement visually, by looking through, with the aim of eliminating it.
After this operation, it is necessary to have the apparatus produce a sample test print, so as to allow, as an example, a data processor to memorize the eventual displacement remaining between the registration marks of the printing support and the registration marks of the printing frame, so as to subsequently send to the stepper motors provided on the printing frame impulses suitable to position the latter so as to eliminate the existing displacement.